The use of data transmission networks based upon packet switching becomes more and more important today for transmitting data within a delimit ed area such as a firm, a plant, a building, etc. through Local Area Networks (LANs) as for transmitting data over long distances by using Wide Area Networks (WANs).
In such a network, when one host has a large amount of data to send to another host, the data are transmitted as a series of datagrams or packets. It is usually preferable that these datagrams be of the largest size that can successfully traverse the path from the source node to the destination node without requiring being fragmented anywhere along the path. This datagram size is referred to as the path Maximum Transmission Unit (MTU) and is equal to the minimum link MTU of all the links in the path. Although there are several techniques for dynamically discovering this path MTU, the current practice is to use the first hop MTU when the first hop is a low speed link like a remote access where generally a MTU value of 576 bytes is used.
The fragmentation of the frames meeting the MTU, required by the low transmission speed on some links of the path, presents some advantages. Thus, in the data transmission systems with acknowledgments, such as TCP/IP systems, the transmission of short frames enables them to be presented (generally displayed) early in time to the user. Furthermore, long frames are not well or not all supported by some switching or routing equipment, or even certain applications, when a low speed link in the remote access results in long transmission delays.
When there are both data and voice transmission on the same path, the voice frames are generally short and do not enable a too important jitter which is produced for example when a long data frame is transmitted between two voice frames. Therefore, the frame size must be reduced to obtain an acceptable jitter to avoid using a too important destination jitter buffer leading to a too long transmission delay incompatible with the requirements of the voice transmission.
But the transmission of short frames also presents a number of drawbacks. Thus, the data processing in a node becomes more complex insofar as for a given quantity of transmitted data, the number of frames which are stored and processed is increased. Furthermore, the number of frames that a node can process in a given time is limited insofar as there is an incompressible processing time and a buffer is required for each frame. For example, with an usual MTU of 576 bytes, buffers of 1 Kbytes are necessary since buffers of 512 bytes are not sufficient, and buffering is not optimized. Another drawback which is not the least is that the use of short frames requires important headers with respect to the data which results in an overhead that can represent 10% of the data.